1. Field of the Invention
The present invention relates to a hydraulic regenerative braking system and method for a vehicle.
2. Background Art
It is well known that hydraulic regenerative systems promise improved efficiency over electric regenerative systems incorporating a battery. Hydraulic regeneration involves using a pump connected in the vehicle drive train as a retarding device, and then storing the resulting high pressure fluid in an accumulator. On subsequent vehicle acceleration, the high pressure fluid from the accumulator is routed to a hydraulic motor and the stored energy is recovered in the form of mechanical work which drives the vehicle forward. A low pressure accumulator acts as a reservoir to make up for fluid volume variations within the high pressure accumulator, and also provides a charge pressure to the inlet side of the pump.
One method of modulating braking and driving forces in hydraulic regenerative systems is to incorporate a variable displacement device to operate in concert with the fixed pressure accumulator. Conventional variable displacement hydraulic machines may vary the piston strokes to achieve the desired power modulation. Such devices can be bulky, heavy and expensive. Moreover, they do not package easily in automotive passenger vehicles, especially in the front of a vehicle, where space is limited.
One way to overcome the limitations associated with conventional variable displacement hydraulic machines is to use a fixed displacement machine. Such a machine is generally smaller and lighter than its variable displacement counterpart, but it does not allow the power modulation required in most applications. One solution to this problem is to use a fixed displacement hydraulic machine in conjunction with a variable ratio hydraulic transformer to facilitate the desired power modulation. One such system is described in U.S. patent application Ser. No. 10/535,354, entitled “Hydraulic Regenerative Braking System for a Vehicle,” filed on May 18, 2005, which is hereby incorporated herein by reference. As an alternative, it would be desirable to have a system that included a relatively compact variable displacement hydraulic machine, thus eliminating the requirement of a separate variable ratio transformer.
Therefore, a need exists for a hydraulic regenerative braking system and method for a vehicle that uses a variable displacement hydraulic machine to provide control of power modulation, without consuming too much space in the vehicle powertrain.